Support lexing and parsing positional params (#7651)

This implements lexing and parsing positional parameters such as `$0`.

toolchain/check/handle_positional_param_expr.cpp

@@ -0,0 +1,14 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/check/context.h"
+#include "toolchain/check/handle.h"
+#include "toolchain/parse/node_ids.h"
+
+namespace Carbon::Check {
+auto HandleParseNode(Context& context, Parse::PositionalParamExprId node_id)
+    -> bool {
+  return context.TODO(node_id, "HandlePositionalParamExprId");
+}
+}  // namespace Carbon::Check